The present invention relates to crystalline propylene copolymer compositions useful in the preparation of heat-sealable films, sheets and films thereof and to a process for preparing said compositions.
Crystalline copolymers of propylene with other olefins (mainly ethylene, 1-butene or both), or mixtures of such copolymers with other olefin polymers are known in the prior art as heat-sealable materials.
These crystalline copolymers are obtained by polymerizing propylene with minor amounts of other olefin comonomers in the presence of coordination catalysts.
The polymerized comonomer units are statistically distributed in the resulting copolymer and the melting point of said copolymers results to be lower than the melting point of crystalline propylene homopolymers. Also the seal initiation temperature (as later defined in detail) of the said copolymers results to be favorably low.
However, the introduction of the comonomer units adversely affects the crystal structure of the polymer, resulting in relatively large quantities of a polymer fraction soluble in organic solvents, so that the copolymers having a particularly low seal initiation temperature cannot be used in the field of food packaging.
Many technical solutions are disclosed in the prior art in order to find a good balance between heat-sealability (as demonstrated by low seal initiation temperatures) and solubility. In particular, published European patent application 483523 discloses compositions prepared directly in a polymerization process, having a low seal initiation temperature and a low content of a fraction soluble in xylene at room temperature or in n-hexane at 50xc2x0 C. These compositions comprise (by weight):
30-60% of a copolymer of propylene and a C4-C8 alpha-olefin, containing 80-98% of propylene;
35-70% of a copolymer of propylene with ethylene and optionally 1-10% of a C4-C8 alpha-olefin, wherein the content of ethylene is 5-10% when the C4-C8 alpha-olefin is not present, or 0.5-5% when the C4-C8 alpha-olefin is present.
Published European patent application 674991 discloses other compositions prepared directly in a polymerization process, having a good ink adhesion in addition to a low seal initiation temperature and low content of a polymer fraction soluble in organic solvents. These compositions comprise (by weight):
20-60% of a copolymer of propylene with ethylene, containing 1 to 5% of ethylene;
40-80% of a copolymer of propylene with ethylene and a C4-C8 alpha-olefin, the ethylene content being 1-5% and the C4-C8 alpha-olefin content being 6-15%;
the total content of ethylene in the compositions being 1-5% and the total content of C4-C8 alpha-olefin in the compositions being 2.4-12%.
Other heat-sealable compositions, comprising two different kinds of copolymers of propylene with higher alpha-olefins, are disclosed in the published European patent application 560326.
On the other hand, it is known that, in general, the Melt Flow Rate (MFR) of an olefin polymer can be adjusted by degradation, in particular by treatment at elevated temperatures in the presence of an initiator of free radicals (xe2x80x9cvisbreakingxe2x80x9d treatment). In the field of heat-sealable olefin copolymers it is also known that the said degradation treatment can improve the heat-seal, optical and solubility properties, as explained in particular in U.S. Pat. No. 5,246,769 and in the Japanese published patent application Sho 59-117506.
However, in these two documents the degradation treatment is applied to single copolymers of propylene with ethylene and/or higher alpha-olefins, resulting into a still high seal initiation temperature.
An attempt to apply the degradation treatment to heat-sealable compositions comprising two copolymers of propylene with ethylene and/or higher alpha-olefins is disclosed in the published European patent application 203727. In this case a copolymer of propylene with 25-45% by weight of butene-1 is first subjected to the degradation treatment and then blended with another copolymer of propylene with ethylene and/or butene-1. However, while the seal initiation temperature and the haze values are brought to satisfactory levels, the content of fraction soluble in organic solvents results to be high (25xc2x0 C. xylene-soluble fraction contents of around 30% are considered to be satisfactory).
It has now surprisingly been found that a particularly valuable balance of heat-sealability, low content of a fraction soluble in organic solvents and optical properties (in particular a very low Haze) is obtained by subjecting specific crystalline propylene copolymer compositions to a degradation treatment. Therefore the present invention provides crystalline propylene copolymer compositions having MFR L values from 2 to 15 g/10 min., preferably from 3 to 15 g/10 min., more preferably from 4 to 10 g/10 min., and comprising (percent by weight):
A) 20-80%, preferably 20-60%, more preferably 30-50%, of one or more propylene copolymers selected from the group consisting of (A 1) propylene/ethylene copolymers containing 1-7% of ethylene; (A 2) copolymers of propylene with one or more C4-C8 alpha-olefins, containing 2-10% of the C4-C8 alpha-olefins; (A 3) copolymers of propylene with ethylene and one or more C4-C8 alpha-olefins, containing 0.5-4.5% of ethylene and 2-6% of C4-C8 alpha-olefins, provided that the total content of ethylene and C4-C8 alpha-olefins in (A 3) be equal to or lower than 6.5%;
B) 20-80%, preferably 40-80%, more preferably 50-70%, of one or more propylene copolymers selected from the group consisting of (B 1) copolymers of propylene with one or more C4-C8 alpha-olefins, containing from more than 10% to 30% of C4-C8 alpha-olefins; (B 2) copolymers of propylene with ethylene and one or more C4-C8 alpha-olefins, containing 1-7% of ethylene and 6-15% of C4-C8 alpha-olefins;
the said MFR L values (MFR L (2)) being obtained by subjecting to degradation a precursor composition comprising the same components A) and B) in the above said proportions, but having MFR L values (MFR L (1)) from 0.3 to 5 g/10 min., preferably from 0.5 to 3 g/10 min., with a ratio MFR L (2) to MFR L (1) of from 2 to 20, preferably from 3 to 15.
From the above definitions it is evident that the term xe2x80x9ccopolymerxe2x80x9d includes polymers containing more than one kind of comonomers.
As previously said, the compositions of the present invention have low seal initiation temperatures (preferably lower than 100xc2x0 C.), a low content of a fraction soluble or extractable in organic solvents (preferably equal to or lower than 20% by weight in xylene at 25xc2x0 C. and equal to or lower than 5% by weight in n-hexane at 50xc2x0 C.), and very low haze values (preferably lower than 1%, more preferably equal to or lower than 0.5%, measured on films according to the method described in the examples).
The melting temperature of said composition is preferably from about 126 to 147xc2x0 C.
Moreover, the compositions of the present invention can be obtained by an efficient and inexpensive process (constituting a further object of the present invention), comprising the following stages:
1) preparing the previously said precursor composition by polymerizing the monomers in at least two sequential steps, wherein components A) and B) are prepared in separate subsequent steps, operating in each step in the presence of the polymer formed and the catalyst used in the preceding step, and dosing the molecular weight regulator (preferably hydrogen) in such amounts as to obtain a MFR L (1) value for the precursor composition of from 0.3 to 5 g/10 min., preferably from 0.5 to 3 g/10 min.;
2) subjecting the precursor composition obtained in 1) to a degradation treatment in order to obtain MFR L (2) values for the final composition from 3 to 15 g/10 min. preferably from 4 to 10 g/10 min., with a degradation ratio, in terms of ratio MFR L (2) to MFR L (1), of from 2 to 20, preferably from 3 to 15.
Such a preferred process is extremely convenient, as it avoids the separate preparation of the components of the precursor composition and separate degradation treatments.
From the preceding description it should be clear that in the precursor composition the comonomer content and relative amounts of components A) and B) are the same as in the final composition (after degradation). The degradation treatment has the effect of increasing the MFR L values of the composition from MFR L (1) to MFR L (2), with the said values of the ratio between the two MFR L values, namely MFR L (2)/MFR L (1), of from 2 to 20.
The above said MFR L values are measured according to ASTM D 1238 L.
In both the precursor and the final compositions the MFR L values of the components A) and B) are not particularly critical, provided that the MFR L values of the total compositions fall within the said ranges.
Indicatively, in the precursor composition the MFR L value of both A) and B) can be from 0.1 to 10 g/10 min.
Preferred comonomer contents in the compositions of the present invention are (by weight):
2-4% of ethylene for (A 1);
4-8% of one or more C4-C8 alpha-olefins for (A 2);
1-4% of ethylene and 2-5% of one or more C4-C8 alpha-olefins, with a total content of ethylene and C4-C8 alpha-olefins equal to or lower than 6%, for (A 3);
11-20% of one or more C4-C8 alpha-olefins for (B 1);
2-4% of ethylene and 7-12% of one or more C4-C8 alpha-olefins for (B 2).
Moreover, when an optimal ink adhesion is desired, the compositions of the present invention should comprise a component (A 1) and a component (B 2) in the previously said relative amounts, while the total content of ethylene in the compositions should be 1-5%, preferably 2-4% by weight and the total content of C4-C8 alpha-olefins in the same compositions should be 2.4-12%, preferably 3.5-8.4% by weight.
Examples of C4-C8 alpha olefins are 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene. Particularly preferred is 1-butene.
As previously explained, the precursor compositions can be prepared by sequential polymerization. Such polymerization is carried out in the presence of stereospecific Ziegler-Natta catalysts. An essential component of said catalysts is a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond, and an electron-donor compound, both supported on a magnesium halide in active form. Another essential component (co-catalyst) is an organoaluminum compound, such as an aluminum alkyl compound.
An external donor is optionally added.
The catalysts generally used in the process of the invention are capable of producing polypropylene with an isotactic index greater than 90%, preferably greater than 95%. Catalysts having the above mentioned characteristics are well known in the patent literature; particularly advantageous are the catalysts described in U.S. Pat. No. 4,399,054 and European patent 45977.
The solid catalyst components used in said catalysts comprise, as electron-donors (internal donors), compounds selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids.
Particularly suitable electron-donor compounds are phthalic acid esters, such as diisobutyl, dioctyl, diphenyl and benzylbutyl phthalate.
Other electron-donors particularly suitable are 1,3-diethers of formula: 
wherein RI and RII are the same or different and are C1-C18 alkyl, C3-C18 cycloalkyl or C7-C18 aryl radicals; RIII and RIV are the same or different and are C1-C4 alkyl radicals; or are the 1,3-diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6 or 7 carbon atoms and containing two or three unsaturations.
Ethers of this type are described in published European patent applications 361493 and 728769.
Representative examples of said dieters are 2-methyl-2-isopropyl-1,3-dimethoxypropane, 2,2-diisobutyl-1,3-dimethoxypropane, 2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane, 2-isopropyl-2-isoamyl-1,3-dimethoxypropane, 9,9-bis(methoxymethyl)fluorene.
The preparation of the above mentioned catalyst components is carried out according to various methods.
For example, a MgCl2.nROH adduct (in particular in the form of spheroidal particles) wherein n is generally from 1 to 3 and ROH is ethanol, butanol or isobutanol, is reacted with an excess of TiCl4 containing the electron-donor compound. The reaction temperature is generally from 80 to 120xc2x0 C. The solid is then isolated and reacted once more with TiCl4, in the presence or absence of the electron-donor compound, after which it is separated and washed with aliquots of a hydrocarbon until all chlorine ions have disappeared.
In the solid catalyst component the titanium compound, expressed as Ti, is generally present in an amount from 0.5 to 10% by weight. The quantity of electron-donor compound which remains fixed on the solid catalyst component generally is 5 to 20% by moles with respect to the magnesium dihalide.
The titanium compounds which can be used for the preparation of the solid catalyst component are the halides and the halogen alcoholates of titanium. Titanium tetrachloride is the preferred compound.
The reactions described above result in the formation of a magnesium halide in active form. Other reactions are known in the literature, which cause the formation of magnesium halide in active form starting from magnesium compounds other than halides, such as magnesium carboxylates.
The active form of magnesium halide in the solid catalyst component can be recognized by the fact that in the X-ray spectrum of the catalyst component the maximum intensity reflection appearing in the spectrum of the nonactivated magnesium halide (having a surface area smaller than 3 m2/g) is no longer present, but in its place there is a halo with the maximum intensity shifted with respect to the position of the maximum intensity reflection of the nonactivated magnesium dihalide, or by the fact that the maximum intensity reflection shows a width at half-peak at least 30% greater than the one of the maximum intensity reflection which appears in the spectrum of the nonactivated magnesium halide. The most active forms are those where the above mentioned halo appears in the X-ray spectrum of the solid catalyst component.
Among magnesium halides, the magnesium chloride is preferred. In the case of the most active forms of magnesium chloride, the X-ray spectrum of the solid catalyst component shows a halo instead of the reflection which in the spectrum of the nonactivated chloride appears at 2.56 xc3x85.
The Al-alkyl compounds used as co-catalysts comprise the Al-trialkyls, such as Al-triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, or SO4 or SO3 groups.
The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.
The electron-donor compounds that can be used as external donors include aromatic acid esters such as alkyl benzoates, and in particular silicon compounds containing at least one Sixe2x80x94OR bond, where R is a hydrocarbon radical.
Examples of silicon compounds are (tert-butyl)2 Si(OCH3)2, (cyclohexyl)(methyl)Si(OCH3)2, (phenyl)2 Si(OCH3)2 and (cyclopentyl)2 Si(OCH3)2. 1,3-diethers having formulae described above can also be used advantageously. If the internal donor is one of these dieters, the external donors can be omitted.
As previously said, the polymerization stage can be carried out in at least two sequential steps, wherein components A) and B) are prepared in separate subsequent steps, operating in each step, except the first step, in the presence of the polymer formed and the catalyst used in the preceding step. The catalyst is added only in the first step, however its activity is such that it is still active for all the subsequent steps.
The order in which components A) and B) are prepared is not critical.
The polymerization stage, which can be continuous or batch, is carried out following known techniques and operating in liquid phase, in the presence or not of inert diluent, or in gas phase, or by mixed liquid-gas techniques. It is preferable to carry out the polymerization in gas phase.
Reaction time, pressure and temperature relative to the two steps are not critical, however it is best if the temperature is from 20 to 100xc2x0 C. The pressure can be atmospheric or higher. The regulation of the molecular weight is carried out by using known regulators, hydrogen in particular.
The catalysts can be pre-contacted with small amounts of olefins (prepolymerization).
Specific examples of precursor compositions suited for the obtainment of the compositions of the present invention by a degradation treatment, and of polymerization processes for preparing the said precursor compositions, are disclosed in the already cited published European patent applications 483523, 560326 and 674991.
The degradation treatment can be carried out by any means and under the conditions known in the art to be effective in reducing the molecular weight of olefin polymers. In particular it is known that the molecular weight of olefin polymers can be reduced by application of heat (thermal degradation), preferably in the presence of initiators of free radicals, like ionizing radiations or chemical initiators.
Particularly preferred among the chemical initiators are the organic peroxides, specific examples of which are 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl-peroxide.
The degradation treatment with the chemical initiators can be carried out in the conventional apparatuses generally used for processing polymers in the molten state, like in particular single or twin screw extruders. It is preferred to operate under inert atmosphere, for instance under nitrogen.
The amount of chemical initiator to be added to the precursor composition can be easily determined by one skilled in the art, based upon the MFR L (1) value (i.e. the MFR L value of the precursor composition) and the desired MFR L (2) value (i.e. the MFR L value of the final composition). Generally such amount is comprised in the range of from 100 to 700 ppm.
The degradation temperature is preferably in the range of from 180 to 300xc2x0 C.
The compositions of the present invention can also contain additives commonly employed in the art, such as antioxidants, light stabilizers, heat stabilizers, colorants and fillers. Among the various applications made possible by the previously described properties, the compositions of the present invention are particularly useful for the preparation of films and sheets.
Films are generally characterized by a thickness of less than 100 xcexcm, while sheets have generally a thickness greater than or equal to 100 xcexcm.
Both films and sheets can be mono- or multilayer.
In the case of multilayer films or sheets, at least one layer comprises the compositions of the present invention. Each layer that does not comprise the compositions of the present invention can be composed of other olefin polymers, such as polypropylene or polyethylene. Generally speaking, the films and sheets of this invention can be prepared by known techniques, such as extrusion and calendering. Specific examples of films containing the compositions of the present invention are disclosed hereinafter in the test for determining the seal initiation temperature (S.I.T.).